feat: warn when main.cpp causes ino preprocessing to be skipped #650

Context

The source scanner currently skips .ino preprocessing whenever a main.cpp file exists. The code comment describes this as supporting projects where main.cpp includes .ino files manually, but the implementation only checks for the presence of main.cpp; it does not verify that the .ino files are included.

This can surprise users because .ino files may be silently ignored.

Relevant area:

• crates/fbuild-build/src/source_scanner.rs

Proposal

When both main.cpp and one or more .ino files are present, fbuild should emit a hard warning in yellow explaining that main.cpp is being used and automatic .ino preprocessing is being skipped.

The compile should still proceed.

Acceptance criteria

• When main.cpp and .ino files are both present, fbuild prints a yellow warning.
• The warning clearly states that main.cpp takes precedence and .ino preprocessing is skipped.
• The compile still proceeds after the warning.
• Tests cover the warning behavior.
• No warning is emitted for main.cpp-only projects or .ino-only projects.

Open questions

• Should fbuild later verify that main.cpp actually includes the .ino files, or is the warning sufficient?